Fiber laser sensor for measuring differential pressures and flow velocities

ABSTRACT

The present invention relates to a fiber laser pressure sensor  1  which is suitable in particular for measuring differential pressures and flow velocities v 1  in oil boreholes. The fiber laser  2  according to the invention comprises two sensor fiber segments  5   a   , 5   b  subjected to different pressure loading, in which segments a birefringence proportional to the differential pressure Δp=p 1 −p 2 , and consequently a beat frequency, is induced between the polarization modes or spatial modes in the fiber laser  2 . Exemplary embodiments with polarimetric monomode fibers  5   a   , 5   b  and/or with elliptical two-mode fibers  5   a   , 5   b  are specified. Furthermore, pressure-resistant multichamber sensor housings  25  and wavelength division multiplex arrangements are disclosed for the fiber laser pressure sensor  1 . One advantage is that the pressure signal is wavelength-coded and thus highly insensitive to interference. It can be read out directly fiber-optically over large distances between the passively optical sensor head  1  and the optoelectronic measuring device  12, 17 . One application concerns the measurement of a flow velocity v 1  with the aid of a Venturi tube  23.

FIELD OF THE INVENTION

The present invention relates to the field of optical pressuremeasurement. It is based on a fiber-optic laser according to thepreamble of claims 1 and 14.

BACKGROUND OF THE INVENTION

In oil production it is necessary to monitor boreholes with regard topressure and temperature. ID the borehole, the liquid pressures can beup to approximately 100 MPa (1000 bar) and the temperatures can be up toabove 200° C. Electrical sensors, such as e.g. piezoresistors,piezoelectric elements, capacitive probes or crystal resonators, oroptical pressure sensors, such as e.g. Fabry-Perot resonators, orelasto-optical sensors, are often used for pressure measurement up toapproximately 170° C.

A polarimetric fiber laser sensor in accordance with the preamble isdisclosed in the article by H. K. Kim et al., “Polarimetric fiber lasersensors”, Optics Letters 18 (4), pp. 317-319 (1993). An Nd-doped fiberwith a round core and dichroically mirrored ends which are transparentto pumping light is used as laser and pressure sensor fiber.Unidirectional lateral pressure on the fiber induces a birefringence andthus a frequency shift between the linear inherent polarizations oflongitudinal modes. The resulting beat frequency in the polarimetricinterference signal can be measured very easily using a frequencycounter.

In accordance with the article by G. A. Ball et al., “Polarimetricheterodyning Bragg-grating fiber-laser sensor”, Optics Letters 18 (22),pp. 1976-1978, instead of dichroic mirrors, it is also possible to usetwo fiber Bragg-gratings written directly into the fiber core for thepurpose of bounding the laser cavity.

In both arrangements, hydrostatic or all-round isotropic pressurescannot be measured. The measurement of steady-state or absolutepressures is difficult or impossible since the operating point, i.e. thebeat frequency in the unloaded state, can fluctuate in an uncontrolledmanner as a result of temperature fluctuations, changes in opticalparameters owing to material fatigue and the like.

Serial multiplexing of passive fiber Bragg-grating sensors is disclosede.g. in U.S. Pat. No. 4,761,073. A plurality of fiber Bragg-gratingswith different reflection wavelengths are written in along a sensorfiber. Each grating can be read out wavelength-selectively and/or bymeans of time-resolved measurement with a pulsed light source.

U.S. Pat. No. 5,515,459 discloses a fiber-optic pressure sensor formeasuring an isotropic pressure, the sensor fiber having two side-holefiber segments with a structure that is not rotationally symmetrical.The two fiber segments are arranged such that they are rotated by 90°with respect to one another, and are exposed to the same isotropicpressure, the side holes of one fiber segment being exposed to theisotropic pressure, but the side holes of the other fiber segment notbeing so exposed. In this case, the rotation of the two fiber segmentsby 90° serve for temperature compensation.

J. P. Darkin and C. Wade, “Compensated polarimetric sensor usingpolarization-maintaining fiber in a differential configuration”,Electron. Lett. Vol. 20, No. 1, pages 51-53, 1984, disclose afiber-optic temperature sensor in which two fiber segments are arrangedsuch that they are rotated by 90° with respect to one another, in orderto compensate common-mode temperature changes and isotropic pressurechanges.

SUMMARY OF THE INVENTION

It is an object of the present invention to specify a fiber laserpressure sensor which is suitable for measuring differential pressuresin liquids or gases and is distinguished by a simple construction, agood measurement sensitivity and a large measurement range. This objectis achieved according to the invention by means of the features ofclaims 1 and 14.

The invention consists in arranging in the laser cavity of a fiberlaser, in addition to the laser-amplifying fiber, two sensor fibersegments which are not rotationally symmetrical and have a mutuallyopposite pressure dependence of the birefringence, and in providingmeans for determining a birefringence-induced beat frequency. By virtueof the rotational asymmetry, an isotropic pressure is converted into ananisotropic birefringence in the fiber laser. The mutually oppositepressure dependence has the effect that the total birefringence andhence the induced beat frequency is proportional to the pressuredifference at the fiber segments.

One exemplary embodiment shows a fiber laser differential pressuresensor with two polarimetric sensor fiber segments. By virtue of 90°rotation between the segments, it is possible to achieve a couplingbetween the polarization modes and, as a result, an opposite pressuresensitivity and inherent compensation of temperature effects.

Another exemplary embodiment shows a fiber laser differential pressuresensor with two spatially bimodal sensor fiber segments. The oppositepressure sensitivity and temperature compensation can be achieved bycoupling between the spatial modes at a transversely offset splicebetween the segments.

A further exemplary embodiment represents a serial arrangement of aplurality of fiber laser differential pressure sensors with differentemission wavelengths, which are fed via a common pumping light sourceand whose pressure-proportional beat frequencies are detected in awavelength-selective manner.

Additional exemplary embodiments relate to pressure housings for fiberlasers, in the case of which the sensor fiber segments are in pressureexchange with two media and, if appropriate, the laser-amplifying fiberand the fiber Bragg-gratings effective as laser mirrors are shieldedfrom the pressures.

Further embodiments, advantages and applications of the invention emergefrom the dependent claims and from the following description withreference to the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures, for a differential pressure sensor according to theinvention:

FIG. 1 shows a polarimetric fiber laser differential pressure sensor;

FIG. 2 shows examples of sensor fibers that are not rotationallysymmetrical;

FIG. 3 shows an alternative polarimetric detection unit;

FIG. 4 shows a transversely bimodal fiber laser differential pressuresensor;

FIG. 5 shows a multiplex arrangement in reflection with a plurality offiber laser differential pressure sensors with different emissionwavelengths;

FIGS. 6-8 show different pressure housings for a fiber laserdifferential pressure sensor; and

FIG. 9 shows a Venturi tube with differential pressure sensor fordetermining flow velocities.

In the figures, identical parts are provided with identical referencesymbols.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to FIG. 1, a fiber laser differential pressure sensoraccording to the invention comprises a pumping light source 8, feedfibers 7 a-7 d, a fiber laser 2, which has a laser-amplifying fiber 3and at least two end reflectors 4 a, 4 b, a detection unit 12 and anelectronic evaluation unit 17. At least two sensor fiber segments 5 a, 5b with a structure that is not rotationally symmetrical and is therebyanisotropically pressure-sensitive are arranged between the endreflectors 4 a, 4 b. The sensor fiber segments 5 a, 5 b have an oppositepressure sensitivity of the birefringence and the detection unit 12 andelectronic evaluation unit 17 comprise means for determining abirefringence-induced beat frequency. The beat frequency is thus largelyproportional to the difference between the pressures p₁ and p₂ which acton the two sensor fiber segments 5 a, 5 b of the fiber laser 2. Theopposite or differential pressure sensitivity can be realized, inparticular, by the fact that the sensor fiber segments 5 a, 5 b carry aplurality of polarization modes and/or spatial modes and an opticalcoupling with mode exchange is provided between the sensor fibersegments 5 a, 5 b. The sensor fiber segments 5 a, 5 b are thus opticallyconnected to one another in such a way that their fiber modes(polarization modes, spatial modes) are differentially coupled, i.e.cross-coupled.

The sensor fiber 5 a, 5 b is typically birefringent, in particularstrongly birefringent or polarization-maintaining. A monomode fiber(FIG. 1) acting as a polarimeter or a fiber with an elliptical core(FIG. 7) which can be used as a two-mode fiber are highly suitable. Inthe reflection configuration illustrated, the pumping light source 8,the fiber laser 2 and the detection unit 12 are optically connected bymeans of a preferably wavelength-selective fiber coupler 9. Thedetection unit 12 has means for detecting a beat frequency in thereceived light from the fiber laser 2, in particular an activepolarization monitoring arrangement 13, an analyzer 14 and a detector 15for the light transmitted by the fiber laser pressure sensor 1. As analternative to the polarization monitoring arrangement 13, thecomponents 7 b-7 d, 9, 11 may be designed such that they arepolarization-maintaining, and be used with same orientation as theanalyzer 14. As a further alternative, the beat signal can be generateddirectly outside the fiber laser 2 by a fiber polarizer 14 (notillustrated) which is oriented at an angle ≈0° or 90°, in particular at45°, to the birefringence axes of the sensor fiber 5 a, 5 b. Thedetector 15 is connected via a signal line 16 to the electronicevaluation unit 17, which, for its part, has means for determining thebeat frequency, in particular a frequency filter 18 and a frequencycounter 19. The fibers 3, 4 a, 4 b, 5 a, 5 b are typically connected toone another and to the feed fibers 7 b, 7 d by splices 10 a-10 f andform a mechanically stable construction. An optical isolator 11 betweenthe fiber coupler 9 and the detection unit 12 is useful for suppressingretroreflections into the fiber laser 2.

The rotationally asymmetrical structure of the sensor fiber segments 5a, 5 b serves for creating an anisotropy for converting the isotropicpressures p₁, p₂ of two fluids 29 a, 29 b into a birefringence of thesensor fiber 5 a, 5 b. In addition to fibers with an elliptical core(shape-induced birefringence), those having a “bow-tie” or “panda”structure (stress-induced birefringence) are also customary. These typesare presented in the article by K.-H. Tsai et al., “General Solutionsfor Stress-Induced Polarization in Optical Fibers”, Journal of LightwaveTechnology Vol. 9, No. 1, 1991. FIG. 2 shows, as further examples ofbirefringent sensor fibers 5 a, 5 b, a fiber (A) with an elliptical orround core 51 and ground cladding 52 (“D-shape” structure), a fiber (C)with an elliptical core and an elliptical fiber cladding and a fiber (D)with a round core 51, a round cladding 52 and side holes 53 (“side-hole”structure). A special feature is represented by the fiber (B)—free frombirefringence in the unloaded state—with a round core 51 and a partiallyground, rotationally asymmetrical fiber cladding 52. The fiber cladding52 can be ground on one side, two sides or a plurality of sides, withthe result that isotropic pressure cancels the degeneracy of thepolarization modes and induces birefringence.

The sensor fiber segments 5 a, 5 b advantageously carry precisely twopolarization modes (polarimetric fibers) and are rotated by 90° withrespect to one another or they carry precisely two spatial modes(two-mode fiber) and are rotated by 0° with respect to one another. Inparticular, the sensor fiber segments 5 a, 5 b are of the same type andhave the same length. As a result, a temperature dependence of thestatic and pressure-induced birefringence can inherently be compensated.Furthermore, an intermediate fiber 30 may be arranged between the sensorfiber segments 5 a, 5 b.

The end reflectors are preferably fiber Bragg-gratings 4 a, 4 b. Inparticular, the Bragg wavelength λ_(B) is a measure of the temperature.In order to determine the temperature-induced Bragg wavelength shift(approximately 10.3 pm/° C. at 1550 nm), received light is branched offe.g. using a fiber coupler and is analyzed in a spectrometer (notillustrated). The fiber Bragg-gratings 4 a, 4 b can be written directlyinto the laser-amplifying fiber 3 and/or into the sensor fiber segments5 a, 5 b. In order to improve the insensitivity to thermal drift, thefiber Bragg-gratings 4 a, 4 b can be chosen with a different bandwidth,preferably Δv_(B) ⁽¹⁾=0.6 nm and Δv_(B) ⁽²⁾=0.2 nm. The reflectivitiesof the fiber Bragg-gratings 4 a, 4 b are typically 99% and 90%. Insteadof fiber Bragg-gratings 4 a, 4 b it is possible to provide reflectivedichroic layers.

FIGS. 1 and 3 show arrangements for generating and detecting the beatfrequency(-ies) for polarimetric sensor fibers 5 a, 5 b. The lightemitted by the fiber laser 2 is (predominantly) coupled out on the sideof the less reflective fiber Bragg-grating 4 a (FIG. 1, reflectionconfiguration) or 4 b (transmission configuration not illustrated). Inthe fiber coupler 9, the emission light is separated from the pumpinglight on account of the shifted wavelength. Retroreflections into thefiber laser 2 are suppressed by the preferably fiber-optic isolator 11and by an obliquely ground portion of the end of the fiber 7 d. In thedetection unit 12, the linear polarization modes x,y are made tointerfere by the analyzer 14. The orientation angle of the analyzer 14relative to the axes of the polarization modes x,y is preferably 45°.The analyzer 14 may be embodied solid-optically or more simply as afiber polarizer 14. In the detector 15, the interference signal isconverted into an intensity-proportional electrical signal. The detector15, typically a photodiode 15, requires for this purpose a bandwidthwhich is greater than the beat frequency to be measured. In thefrequency filter 18, the desired beat signal is separated and fed to afrequency counter 19. Alternative embodiments of the electronicevaluation unit 17 may comprise a radio-frequency spectral analyzer, anoscilloscope or other high-frequency or microwave measuring instruments.

FIG. 3 shows an alternative beat frequency detection according to theinvention. In this case, the detection unit 12 comprises, for thepurpose of splitting, polarization analysis and detection of thereceived light from a fiber laser 2, a polarization-maintaining fibercoupler 22 with, on the output side, two analyzers 14 a, 14 b orientedat 0° and 45°, and two detectors 15 a, 15 b. The analyzers arepreferably fiber polarizers 14 a, 14 b, which are connected to the fibercoupler 22 via splices 21 a, 21 b. The detectors 15 a, 15 b areconnected via signal lines 16 a, 16 b to a summer 20, which is connectedto a frequency filter 18 and a frequency counter 19 for determining thebeat frequency. The beat frequency then represents a highly precisemeasure of the pressure difference Δp=p₁−p₂ between the two sensor fibersegments 5 a, 5 b of the fiber laser 2. A further alternative forgenerating interference between the polarization modes consists inproducing a strong coupling between them, for example by microbending ofthe fiber downstream of the optical isolator 11.

FIG. 4 represents a greatly simplified variant with respect to FIG. 1,in the case of which a spatially bimodal sensor fiber 5 a, 5 b with anelliptical core is used instead of a spatially monomodal one. Theinterference between the spatial modes LP₀₁ and LP₁₁ ^(rectilinear) isformed inherently on account of identical directions of polarization ofthe spatial modes. This obviates the need for a polarization monitoringarrangement 13 and an analyzer 14 upstream of the detector 15. Amutually oppositely identical pressure sensitivity and an inherenttemperature compensation can be realized by means of parallel alignment(0° angle of rotation) of bimodal sensor fiber segments 5 a, 5 b of thesame length and of the same type and by means of a transverse offset ofthe fiber ends in the splice 6′. A fiber polarizer (not illustrated) isadvantageously arranged and in particular inserted with splices, in orbeside the fiber laser 2. The spatial modes LP₀₁, and LP₁₁^(rectilinear) can then build up oscillations only with a linearpolarization and the number of beat frequencies is halved. Inparticular, the Bragg wavelength λ_(B) is chosen in the spectral regionof a vanishing group refractive index birefringence of the sensor fiber5 a, 5 b in order to repress the influence of a temperature dependenceof the birefringence. The sensor construction in accordance with FIG. 4is distinguished by a distinctly reduced complexity and by being verywell suited to multiplex arrangements in accordance with FIG. 5.

FIG. 5 shows a multiplex arrangement which comprises a plurality offiber lasers 2 of different emission wavelengths λ_(l), . . . , λ_(i).The fiber lasers 2 are optically connected to a common pumping lightsource 8 and a detection unit 12. The detection unit 12 has a wavelengthdivision demultiplexer 23 and a multichannel detector 24, which iselectrically connected to a multichannel electronic evaluation unit 17.A beat frequency is detected in each channel in the manner describedabove. In particular, the electronic evaluation unit 17 comprises afrequency filter 18 and a frequency counter 19 for each fiber laser 2.For the serial multiplex arrangement illustrated, the dopingconcentrations and lengths of the laser-amplifying fibers 3 are chosensuch that, in each fiber laser 2, sufficient pumping power is absorbedand sufficient pumping power is transmitted for the subsequent fiberlasers 2. By using separate amplifying fibers 3 and sensor fibers 5 a, 5b, the laser behavior and the pressure sensitivity of the sensor 1 canbe optimized independently of one another. The spacing between theemission wavelengths is chosen to be large enough that the reflectionspectra of all the fiber lasers 2 remain free from overlap even atdifferent temperatures, and spectral separation of the signals in thedemultiplexer 23 is possible. Consequently, each fiber laser 2 requiresa wavelength window of at least 2.4 nm for a temperature range between0° C. and 230° C. The multiplex arrangement can also be constructed inparallel. The directions of propagation of pumping light and laseremission are allowed to have the same direction. A wavelength divisionmultiplex arrangement has the advantage that the fundamentalconstruction of the sensor 1 (FIGS. 1, 4), in particular the reflectionconfiguration with a fiber coupler 9 that selects the pumpingwavelength, can be preserved and the channel separation can be carriedout in a simple manner using the optical wavelength divisiondemultiplexer 23.

The pumping light source 8, the absorption bands of the laser-amplifyingfiber 3 and the Bragg wavelengths λ_(B) of the fiber Bragg-gratings 4 a,4 b are to be spectrally coordinated with one another in such a way thatthe laser threshold for relatively few longitudinal modes in the fiberlaser 2 is as low as possible. For the two polarization or spatialmodes, the longitudinal natural frequencies and, from the latter, thefrequency spacings Δv₀, Δv₀′ between adjacent longitudinal modes can bedetermined in a known manner. For sensor fiber segments 5 a, 5 b of thesame type and of the same length, the following holds true, disregardingdispersion effects,

Δv ₀ =Δv ₀ ′=c/[2·(n _(d) ·L _(d)+(n _(a) +n _(b))·L)],  (E1)

where n_(d)=effective refractive index of the doped fiber 3;L_(d)=length of the doped fiber; n_(a), n_(b)=effective refractiveindices of the coupled-in mode in the two sensor fiber segments 5 a, 5b, L=length of the sensor fiber segments 5 a, 5 b and c=speed of lightin a vacuum. The birefringence is of opposite, equal magnitude in thetwo segments 5 a, 5 b and the frequencies of the two modes aredegenerate. The action of different pressures p₁ and p₂ on the segments5 a and 5 b induces a mutually opposite birefringence in said segments.This results in a total phase shift

Δφ_(tot) =K _(p) ·Δp·L,  (E2)

where K_(p)=proportionality constant, Δp=p₁−p₂ pressure difference. Thisexpression is valid in first order and does not take account ofhigher-order dependencies on pressure or temperature. The naturalfrequencies of orthogonal modes of the fiber laser 2 are shiftedrelative to one another by the phase shift Δφ_(tot). This results indifferential-pressure-proportional beat frequencies Δv₁ and m·Δv₀±Δv₁,m=1, 2, 3, . . . An unambiguous pressure measurement range is given bythe condition Δv₁<Δv₀/2 or vΔφ_(tot)<π/2.

For illustration, some quantitative estimations: the required length ofa polarimetric monomode sensor fiber 5 a, 5 b with an elliptical core(K_(p)=9.5°/(MPa·m) at λ=1548 nm of the fiber laser 2) is 9.5 cm for adifferential pressure measurement range of 100 MPa. The frequency andwavelength spacings between the longitudinal modes result from L_(d)=30cm, L=10 cm and n_(d)=1.45 as 207 MHz and 1.65 pm. Thus, only few modesare supported by the narrowband fiber Bragg-gratings 4 a, 4 b and thebeat frequencies are hardly widened (line width approximately 10 kHz)and the pressure resolution is very high (approximately 4.8 kPa).

For a transversely bimodal sensor fiber 5 a, 5 b with an elliptical core(K_(p)=48.9°/(MPa·m) at λ=1300 nm of the fiber laser 2) the followingare found analogously: required length of 1.84 cm for measurement rangeof 100 MPa and resolution of 0.94 kPa with a line width of 10 kHz.Moreover, “bow-tie” or “panda” fibers are suitable as bimodal sensorfibers 5 a, 5 b.

In principle, the differential pressure sensor 2 can also be constructedfrom a polarimetric monomode sensor fiber segment 5 a and a transverselybimodal segment 5 b.

As is known, dopings with erbium³⁺ (1530 nm-1570 nm; concentrationapproximately 100 ppm-1000 ppm; pumping light at 1480 nm or 980 nm),praseodymium³⁺ (approximately 1300 nm), neodymium³⁺ (approximately 1060nm) or thulium³⁺ (approximately 810 nm) can be used for thelaser-amplifying fiber 2. Advantages of erbium doping include the verylow optical losses of pumping light and induced laser light in the feedfibers 7 a-7 d, the permissibility of long lead fibers 7 a-7 d of up toseveral kilometers and the capability of using standard opticalcomponents for the 1550 nm telecommunications window.

FIGS. 6-8 show exemplary embodiments of pressure housings 25 for thefiber laser differential pressure sensor 2. By means of the transducers2 or pressure housings 25, the end reflectors 4 a, 4 b and, ifappropriate, the laser-amplifying fiber 3 are shielded from an externalpressure and the oppositely pressure-sensitive sensor fiber segments 5a, 5 b are exposed to different pressures p₁, p₂.

For this purpose, the pressure-resistant housings 25 are configured witha plurality of pressure chambers 27 a-27 d connected by pressure-tightfiber feedthroughs 28 a-28 e, for accommodating a fiber laser 2. Thefibers 3, 4 a, 4 b, 5 a, 5 b are fastened in a manner free of stress inthe pressure chambers 27 a-27 d, in order to avoid fiber strain due tothermal or mechanical loading on the housing 25. Separate pressurechambers 27 b, 27 c are provided for the sensor fiber segments 5 a, 5 band, via separate inlets 26 a, 26 b, are in direct pressure exchangewith at least two media 29 a, 29 b. In particular, additional pressurechambers 27 a, 27 d are provided for fiber Bragg-gratings 4 a, 4 b. Thelaser-amplifying fiber 3 can be arranged in an additional pressurechamber 27 a, 27 d, preferably together with a fiber Bragg-grating 4 a,4 b (FIGS. 6, 8). The laser-amplifying fiber 3 may also comprise thesensor fiber segments 5 a, 5 b (FIGS. 7a, 7 b) and have, in particular,at least one fiber Bragg-grating 4 a, 4 b (FIG. 7b). As a result, thelaser cavity 2 can be shortened, the differential pressure resolutioncan be increased, and the laser threshold can be lowered.

The housing 25 is typically elongate, in particular cylindrical. Thediameter should be a maximum of 10 mm. The length is essentiallypredetermined by the length of the fiber laser 2. Instead of the inlets26 a, 26 b, the housing 25 may have an opening with a pressure diaphragm(not illustrated) for pressure exchange between the media 29 a, 29 b anda fluid, for example silicone oil, in the transducer 2. In this way, theambient pressures p₁, p₂ are transmitted all around onto the sensorfiber segments 5 a, 5 b, and the latter are protected against directcontact with the media 29 a, 29 b.

The housing 25 is composed, for example, of corrosion-resistant steel orquartz glass. An internal coating e.g. with gold can reduce theindiffusion of hydrogen. In the case of a single fiber laser 2 or thelast in a multiplex arrangement, lead fiber 7 d and fiber feedthrough 28d are omitted and the fiber with the fiber Bragg-grating 4 b can endwithin the housing 25. The shielded pressure chambers 27 a, 27 d arepreferably under a vacuum, low-pressure gas or normal pressure.

The housing 25 serves only as mechanical protection for the fibers 3, 4a, 4 b, 5 a, 5 b and for spatial separation of the media 29 a, 29 b withthe pressures p₁ and p₂. It does not itself participate in the sensormechanism. The fiber anchorings are not critical. The housing cantherefore be designed for large mechanical and thermal loads in a verysimple manner. It is furthermore distinguished by compactness and a lowweight and is therefore very well suited to use in large fiber links 7a-7 d with many pressure measuring points, e.g. in oil productionboreholes. A fiber sheath which is suitable for high temperatures and ismade e.g. of polyimides or metal, and/or a fiber cable may also beprovided for the fibers 3, 4 a, 4 b, 5 a, 5 b in the housing 25.

FIG. 8 shows a monomode intermediate fiber 30 in the region of thepressure-tight fiber feedthrough 28 c between the fiber segments 5 a, 5b. The intermediate fiber 30 serves for absorbing the force of the fiberfeedthrough 28 c and for enabling the sensor fibers 5 a, 5 b to beretained largely without any forces. The intermediate fiber 30preferably has an elliptical core which ensures a fixed axialorientation, largely independent of force, of the birefringence. Inorder inherently to compensate pressure-dictated phase shifts in theintermediate fiber 30, the polarization or spatial modes (LP₀₁, LP₁₁^(rectilinear)) of the sensor fiber segments 5 a, 5 b are transmitted bythe same mode or modes of the intermediate fiber 30. For this purpose,the core ellipse of the intermediate fiber 30 is oriented atapproximately 45° in the case of polarimetric monomode sensor fibersegments 5 a, 5 b and approximately parallel or orthogonally to the axesof the segments 5 a, 5 b in the case of spatial two-mode sensor fibers 5a, 5 b. In the latter case, the additional splices 61, 62 are offsettransversely in order that the two spatial modes of the segments 5 a, 5b are coupled in and out as uniformly as possible.

In all of the exemplary embodiments, the order of the fibers 3, 5 a, 5 bin the fiber laser 2 can be chosen as desired. In particular, thelaser-amplifying fiber 3 can also be arranged at the rear end of thefiber laser 2 or between the two sensor fiber segments 5 a, 5 b.

FIG. 9 shows a use of a fiber laser differential pressure sensor 1, 25according to the invention, in the case of which a flow velocity v, of afluid flow 24 is determined from a differential pressure measurement. Inparticular, the inlets 26 a, 26 b of the transducer 1 are connected to aVenturi tube 23 at two points with cross-sectional areas A₁ and A₂. Theflow velocity v₁ can be determined from the differential pressureΔp=p1−p2 in a known manner.

Important advantages of the fiber laser differential pressure sensor 1disclosed concern: a high measurement accuracy by virtue of thefrequency coding of the pressure signal; capability of calibration toabsolute pressures and inherent temperature compensation by virtue ofthe differential coupling of the sensor fiber segments 5 a, 5 b; apassively optical sensor head and a fiber-optic signal transmission overlarge distances; and the capability of using commercially availablecomponents, in particular erbium-doped amplifier fibers and two-modesensor fibers with an elliptical core, whose optical properties can beoptimized independently of one another.

LIST OF REFERENCE SYMBOLS

1 Fiber laser differential pressure sensor

2 Fiber laser

3 Laser-amplifying fiber, doped fiber

4 a, 4 b End reflectors, fiber Bragg-grating

5 a, 5 b Birefringent sensor fiber, sensor fiber segments; fibers withan elliptical core (monomode or bimodal)

51 Fiber core

52 Fiber cladding

53 Side holes

6 90° splice

6′ Splice with a transverse offset; 0° splice

61, 62 Splices for intermediate fiber

7 a-7 d Feed fibers

8 Pumping light source, pumping laser

9 Fiber coupler, wavelength division multiplexer

10 a-10 e Splices

11 Optical isolator

12 Detection unit

13 Polarization monitoring arrangement

14, 14 a, 14 b Analyzer, fiber polarizer

15 Detector, photodiode

16 Signal line

17 Electronic evaluation unit

18 Frequency filter

19 Frequency counter

20 Summer

21 a, 21 b 0°, 45° splices

22 Polarization-maintaining fiber coupler

23 Wavelength division demultiplexer

24 Multichannel detector

25 Housing

26 a, 26 b Inlets

27 a-27 d Pressure chambers, housing compartments

28 a-28 e Pressure-tight fiber feedthroughs

29 a Medium 1, fluid 1 (under pressure p₁)

29 b Medium 2, fluid 2 (under pressure p₂)

30 Intermediate fiber

31 Fluid flow

32 Venturi tube

A₁, A₂ Cross-sectional areas

c Speed of light in a vacuum

L Length of the sensor fiber segments

L_(d) Length of the laser-amplifying fiber

λ_(B) Bragg wavelength

λ_(l), . . . , λ_(i) Emission wavelengths

Δv_(B) ⁽¹⁾, Δv_(B) ⁽²⁾ Bandwidths of the fiber Bragg-gratings

Δv₀, Δv₀′ Frequency spacing between longitudinal modes

Δv₁ Fundamental beat frequency

n_(d), n_(a), n_(b) Effective refractive indices

p₁, p₂ Pressures

Δp Differential pressure

v₁, v₂ Flow velocities

What is claimed is:
 1. A fiber laser differential pressure sensor,Δp=p₁−p₂, between two pressures, p₁ and p₂, particularly suitable forpressure measurement in oil boreholes, comprising a pumping lightsource, feed fibers, a fiber laser, which has a laser-amplifying fiberand at least two end reflectors, a detection unit and an electronicevaluation unit, wherein a) at least two sensor fiber segments with astructure that is not rotationally symmetrical are arranged between theend reflectors, and b) the sensor fiber segments have an oppositepressure sensitivity of the birefringence, c) a first pressure p₁ actingon a first sensor fiber segment, and a second pressure p₂ acting on asecond sensor fiber segment, and d) the detection unit and electronicevaluation unit comprise means for determining a birefringence-inducedbeat frequency.
 2. The fiber laser differential pressure sensor asclaimed in claim 1, wherein a) the sensor fiber segments carry aplurality of polarization modes and/or spatial modes and b) an opticalcoupling with mode exchange is provided between the sensor fibersegments.
 3. The fiber laser differential pressure sensor as claimed inclaim 2, wherein a) the detection unit has, for the purpose ofsplitting, polarization analysis and detection of received light from afiber laser, a polarization-maintaining fiber coupler with, on theoutput side, analyzers oriented at 0° and 45° and two detectors, and b)the detectors are connected via a summer to a frequency filter and afrequency counter.
 4. The fiber laser differential pressure sensor asclaimed in claim 1, wherein the sensor fiber segments have an ellipticalcore, a “bow-tie” structure, a “panda” structure, a “side-hole”structure, a “D-shape” structure, an elliptical fiber cladding or apartially ground fiber cladding.
 5. The fiber laser differentialpressure sensor as claimed in claim 1, wherein a) the sensor fibersegments carry precisely two polarization modes and are rotated by 90°with respect to one another, or b) the sensor fiber segments carryprecisely two spatial modes and are rotated by 0° with respect to oneanother.
 6. The fiber laser differential pressure sensor as claimed inclaim 1, wherein the sensor fiber segments are of the same type and havethe same length.
 7. The fiber laser differential pressure sensor asclaimed in claim 1, wherein a) the end reflectors are fiberBragg-gratings, and b) the Bragg wavelength λ_(B) is a measure of thetemperature.
 8. The fiber laser differential pressure sensor as claimedin claim 7, wherein the fiber Bragg-gratings are written directly intothe laser-amplifying fiber and/or into the sensor fiber segments.
 9. Thefiber laser differential pressure sensor as claimed in claim 7, whereinthe fiber Bragg-gratings are chosen with a different bandwidth,preferably Δv_(B) ⁽¹⁾=0.6 nm and Δv_(B) ⁽²⁾=0.2 nm.
 10. The fiber laserdifferential pressure sensor as claimed in claim 1, wherein a) thesensor fiber is spatially bimodal, b) a fiber polarizer is arranged inor beside the fiber laser, and c) in particular the Bragg wavelengthλ_(B) is chosen in the spectral region of a vanishing group refractiveindex birefringence of the sensor fiber.
 11. The fiber laserdifferential pressure sensor as claimed in claim 1, wherein a) thepumping light source, the fiber laser and the detection unit areoptically connected by means of a preferably wavelength-selective fibercoupler, b) the detection unit comprises means for detecting a beatfrequency in the received light from the fiber laser, and c) theelectronic evaluation unit has means for determining the beat frequency.12. The fiber laser differential pressure sensor as claimed in claim 1,wherein a) a plurality of fiber lasers of different emission wavelengthsare optically connected to a common pumping light source and a detectionunit, b) the detection unit has a wavelength division demultiplexer anda multichannel detector, and c) a multichannel electronic evaluationunit is provided.
 13. The fiber laser differential pressure sensor asclaimed in claim 1, wherein a) a pressure-resistance housing with aplurality of pressure chambers connected by pressure-tight fiberfeedthroughs is configured for accommodating a fiber laser, b) separatepressure chambers are provided for the sensor fiber segments, saidpressure chambers being in direct pressure exchange with at least twomedia of the first and second pressures p₁ and p₂, respectively, and c)additional pressure chambers are provided in particular for fiberBragg-gratings.
 14. The fiber laser pressure sensor as claimed in claim13, wherein a) the laser-amplifying fiber is arranged in an additionalpressure chamber, in particular together with a fiber Bragg-grating, orb) the laser-amplifying fiber comprises the sensor fiber segments, andc) the laser-amplifying fiber has at least one fiber Bragg-grating. 15.The use of a fiber-optic differential pressure sensor as claimed inclaim 1, wherein a) a flow velocity v₁ of a fluid flow is determinedfrom a differential pressure measurement, and b) the differentialpressure measurement is carried out on a Venturi tube.
 16. The fiberlaser differential pressure sensor as claimed in claim 1, wherein anintermediate fiber is arranged between the sensor fiber segments.